1. Field of the Invention
This invention relates to the pressure vessel of a pressurized water reactor system of an advanced design and, more particularly, to an improved calandria assembly within the pressure vessel which provides requisite mechanical support functions, taking into account acceptable stress conditions and vibration problems to which the calandria assembly is subjected, while affording enhanced flow conditions.
2. State of the Relevant Art
As is well known in the art, conventional pressurized water reactors employ a number of control rods which are mounted within the reactor vessel, generally in parallel axial relationship, for axial translational movement in telescoping relationship with the fuel rod assemblies. The control rods contain materials which absorb neutrons and thereby lower the neutron flux level within the core. Adjusting the positions of the control rods relative to the respectively associated fuel rod assemblies thereby controls and regulates the reactivity and correspondingly the power output level of the reactor. Typically, the control rods, or rodlets, are arranged in clusters, and the rods of each cluster are mounted to a common, respectively associated spider. Each spider, in turn, is connected to a respectively associated adjustment mechanism for raising or lowering the associated rod cluster.
In certain advanced designs of such pressurized water reactors, there are employed both control rod clusters (RCC) and water displacer rod clusters (WDRC), and also so-called gray rod clusters which, to the extent here relevant, are structurally identical to the RCC's and therefore both are referred to collectively hereinafter as RCC's. In one such reactor design, a total of over 2800 reactor control rods and water displacer rods are arranged in 185 clusters, each of the rod clusters having a respectively corresponding spider to which the rods of the cluster are individually mounted.
In the exemplary such advanced design pressurized water reactor, there are provided, at successsively higher, axially aligned elevations within the reactor vessel, a lower barrel assembly, an inner barrel assembly, and a calandria, each of generally cylindrical configuration, and an upper closure dome. The lower barrel assembly has mounted therein, in parallel axial relationship, a plurality of fuel rod assemblies comprising the reactor core, and which are supported at the lower and upper ends thereof, respectively, by corresponding lower and upper core plates, the latter being welded to the bottom edges of the cylindrical sidewall of the inner barrel assembly. Within the inner barrel assembly there are mounted a large number of rod guides disposed in closely spaced relationship, in an array extending substantially throughout the cross-sectional area of the inner barrel assembly. The rod guides are of first and second types, respectively housing therewithin reactor control rod clusters (RCC) and water displacer rod clusters (WDRC); these clusters, as received in telescoping relationship within their respectively associated guides, generally are aligned with respectively associated fuel rod assemblies.
One of the main objectives of the advanced design, pressurized water reactors to which the present invention is directed, is to achieve a significant improvement in the fuel utilization efficiency, resulting in lower, overall fuel costs. Consistent with this objective, the water displacement rodlet clusters (WDRC's) function as a mechanical moderator control, all of the WDRC's being fully inserted into association with the fuel rod assemblies, and thus into the reactor core, when initiating a new fuel cycle. Typically, a fuel cycle is of approximately 18 months, following which the fuel must be replaced. As the excess reactivity level diminishes over the cycle, the WDRC's are progressively, in groups, withdrawn from the core so as to enable the reactor to maintain the same reactivity level, even though the reactivity level of the fuel rod assemblies is reducing due to dissipation over time. Conversely, the control rod clusters are moved, again in axial translation and thus telescoping relationship relatively to the respectively associated fuel rod assemblies, for control of the reactivity and correspondingly the power output level of the reactor on a continuing basis, for example in response to load demands, in a manner analogous to conventional reactor control operations. The WDRC's provide a mechanical means for spectral shift control of a reactor and a reactor incorporating same is disclosed in the copending application Ser. No. 946,112, filed Dec. 24, 1986 a continuation of application Ser. No. 217,503, filed Dec. 16, 1980 and entitled MECHANICAL SPECTERAL SHIFT REACTOR and applications cited therein; a system and method for achieving the adjustment of both the RCC's and WDRC's are disclosed in the copending application of Altman et al., Ser. No. 806,719, filed Sept. 12, 1985, and entitled "DISPLACER ROD DRIVE MECHANISM VENT SYSTEM." Each of the foregoing applications is assigned to the common assignee hereof and is incorporated herein by reference.
A critical design criterion of such advanced design reactors is to minimize vibration of the reactor internals structures, as may be induced by the core outlet flow as it passes therethrough. A significant factor for achieving that criterion is to maintain the core outlet flow in an axial direction throughout the inner barrel assembly of the pressure vessel and thus in parallel axial relationship relative to the rod clusters and associated rod guides. The significance of maintaining the axial flow condition is to minimize the exposure of the rod clusters to crossflow, a particularly important objective due to the large number of rods and the type of material required for the WDRC's which creates a significant wear potential. This is accomplished by increasing the vessel length sufficiently so as to locate the rods below the vessel outlet nozzles, whereby the rods are subjected only to axial flow. The calandria then is provided as an additional structure, disposed above the inner barrel assembly and thus above the level of the rods. The calandria receives the axial core outlet flow, and turns the flow through 90.degree. to a radial direction for exiting from the radially oriented outlet nozzles of the vessel. The calandria thus must withstand the crossflow generated as the coolant turns from the axial to the radial directions, and must provide for shielding and flow distribution in the upper internals of the vessel. Advanced design pressurized water reactors of the type here considered incorporating such calandria structures are disclosed in the copending application Ser. No. 490,101 to James E. Kimbrell et al., for "NUCLEAR REACTOR"; application Ser. No. 490,059 to Luciano C. Veronesi for "CALANDRIA"; and application Ser. No. 490,099, "NUCLEAR REACTOR" all thereof concurrently filed on Apr. 29, 1983 and incorporated herein by reference. Additionally, structural elements known as formers are included within the vessel to assist in maintaining the desired axial flow condition within the inner barrel, assembly; modular such formers are disclosed in the copending application Ser. No. 798,195, filed Nov. 14, 1985, and entitled "MODULAR FORMER FOR INNER BARREL ASSEMBLY OF PRESSURIZED WATER REACTOR," having a common coinventor herewith and assigned to the common assignee hereof.
In general, the calandria includes a lower calandria plate and an upper calandria plate. The rod guides are secured in position at the lower and upper ends thereof, respectively, to the upper core plate and the lower calandria plate. Within the calandria and extending between aligned apertures in the lower and upper plates thereof is mounted a plurality of calandria tubes in parallel axial relationship, respectively aligned with the rod guides. A number of flow holes are provided in the lower calandria plates, at positions displaced from the apertures associated with the calandria tubes, through which the reactor core outlet flow passes as it exits from its upward passage through the inner barrel assembly. The core outlet flow or a major portion thereof, as received in the calandria, turns from the axial flow direction to a radial direction for passage through radially outwardly oriented outlet nozzles which are in fluid communication with the calandria.
In similar parallel axial and aligned relationship, the calandria tubes are joined to corresponding flow shrouds which extend to a predetermined elevation within the dome, and which in turn are in alignment with and in close proximity to corresponding head extensions which pass through the structural wall of the dome and carry, on their free ends at the exterior of and vertically above the dome, corresponding adjustment mechanisms, as above noted. The adjustment mechanisms have corresponding drive rods which extend through the respective head extensions, flow shrouds, and calandria tubes and are connected to the respectively associated spiders to which the clusters of RCC rods and WDRC rods are mounted, and serve to adjust their elevational positions within the inner barrel assembly and, correspondingly, the level to which the rods are lowered into the lower barrel assembly and thus into association with the fuel rod assemblies therein, thereby to control the reactivity within the core.
The calandria, as before noted, performs the important function of shielding the drive rods and performing flow distribution in the upper internals. Since the radial flow, or crossflow, velocities are the range of 40 feet per second, it must be robust and able to withstand the vibrational loading imposed thereon by such crossflow. Further, the vessel provides a flow path for the coolant to enter the head region directly, for cooling the adjustment mechanisms mounted on the head assembly and vessel dome, and a downcomer flow path through which the head coolant normally passes from the head region to mix with the core outlet flow and exit from the vessel through the outlet nozzles. The head region also serves as a reservoir of low temperature coolant which passes through the downcomer flow path and ultimately into the lower internals, to cool the core in the event of a LOCA (loss of coolant accident). The calandria thus is an interface between the high temperature core outlet flow and the low temperature coolant of the head region, and accordingly is subjected to the significant temperature differential which exists therebetween, and must be flexible in order to limit the magnitude of the resulting thermal stresses.
Conventional reactor internals have no structural analogy to the calandria assembly of such advanced design reactors, and thus there are no known solutions for satisfying the requirements of such a calandria assembly as above set forth and to which the present invention relates.